High-performance computer power supply units, in particular, require electronically commutated fans in order to be cooled. The fan speed is measured electronically for control and adjustment purposes.
Until now, one available option was the utilization of special fans for this purpose. These special fans deliver an internal clock signal of the electronic commutation outward via an additional line. In this case, it is problematic that fans of this type have a significantly higher price such that their utilization should be avoided whenever possible.
It is also known to detect current fluctuations of a fan by means of a shunt, wherein the measuring signal is extracted by means of a filter circuit. A signal that contains the speed information is obtained by means of differentiation and subsequent pulse shaping.
The commutation process can be detected due to the fact that the current changes significantly within a very short period of time when a commutation occurs. The rate of change is higher than the rate of change of the current caused by the back e.m.f. of the motor by at least a factor of 10.
Until now, these commutation pulses were obtained by removing their direct component and shaping the portions of the remaining alternating component with a high rate of change into pulses. A few methods operate with fixed switching thresholds. There exist methods that are carried out with the aid of a capacitor as well as methods that are carried out on a digital basis with D/A converters.
One method that operates on a digital basis with a D/A converter is known from DE 100 21 503 A1. In this case, voltage signals that are proportional to a motor current are evaluated. A comparator that controls an incrementer/decrementer based on a comparison with a threshold value is utilized for this purpose. The count of the incrementer/decrementer then defines the threshold value for the comparator after a digital/analog conversion.
The direct component is removed from the voltage values in this fashion. The commutation signal can be detected based on the sequence of the output signals of the comparator that reflect the alternating component of the voltage values.
However, this method does not function satisfactorily if fans of different types or by different manufacturers should be operated with the same control circuit without having to change adjustments.
In order to allow the monitoring of different fan types that have different operating currents, it is possible to determine the maximum rate of change of the fan current and to utilize a certain percentage thereof as the switching threshold for detecting a commutation. However, this method has serious disadvantages. If the fan becomes jammed, a connected monitoring unit must be able to detect the blocking of the fan. It is absolutely imperative to prevent the incorrect addition of commutation pulses although the fan is not turning. However, this is exactly what occurs in such a detection method, namely as described below. If a commutation no longer takes place due to such a blockage situation, the operating current will cease to fluctuate such that the alternating component drops to zero.
Consequently, the switching threshold for detecting a commutation pulse also drops to a minimum value. If the operating voltage of the jammed fan fluctuates without the fan turning, for example, if other consumers such as hard disks or processors with active energy-saving functions generate abrupt load variations on the same power supply, the current of the jammed fan also fluctuates because it exhibits ohmic characteristics during a blockage.
This leads to the commutation detection circuit incorrectly detecting pulses although the fan is jammed. Consequently, a jammed fan can no-longer be reliably detected and the device to be cooled may quickly overheat.